A mother devastated by the sudden death of her son broke down in tears when her friend said she would run the London Marathon to help her charity.

Diane McDonald's son Gareth died suddenly in his sleep, aged just 27, on February 20, 2000.

His shattered family found he had died of Sudden Death Syndrome (SDS), after suffering a heart condition they had never known about, which caused his heart to stop.

With an unfillable void in her life, Diane became determined to raise money to help other families.

Now her friend Tracy Houghton will run the London Marathon to help fund a heart monitor at Llanrumney Health Centre, which she hopes could help detect heart problems early.

'I just started crying when Tracy told me she wanted to run the marathon for Gareth,' said Diane, a Jobcentre Plus clerical worker, 55, who lives in Taunton Crescent, Llanrumney, Cardiff, with husband Kevin.

'The doctors said Gareth's death was sudden, like a lightbulb that just goes out - there was nothing anyone could have done.

'You're left with a void nothing can fill, it doesn't get easier. Some days I can function, some days I can't. But if I can help one family, that's something.'

Since Gareth's death, Diane has raised £7,380 for the charity Cardiac Risk in the Young (CRY) which supports families who have lost people to SDS.

She now needs just £270 to buy the heart monitor but hopes to raise even more for the charity.

She said: 'Gareth was loving his job as an insurance worker. He had been there since he left St Illytd's High School when he was 17.

'He loved football and was a staunch Liverpool fan and followed Cardiff City.

'We had no idea anything was wrong.'

Six months after Gareth's death a Swansea woman whose son had died of Sudden Death Syndrome told her about CRY.

'I felt like I wasn't on my own and I wanted to help other people,' said Diane.

'Cardiff East rugby team had raised £500 with a memorial game for Gareth at Rumney Rugby Club and it inspired me to set up Gareth's memorial fund with CRY.

'I've made millions of Welsh cakes to raise money, held a charity auction and lots of fundraising events.

'I couldn't have done it without the help of all my colleagues, friends and the people who've supported me.

Jobcentre Plus human resources officer Tracy, 40, of Butleigh Avenue, Victoria Park, Cardiff, who hopes to raise £1,000, said: 'I've seen the major impact Gareth's death has had on Diane and how it changed her. She does so much with the charity.